1. Field of the Invention
The present invention relates to an electric parking brake system provided on such a vehicle as an automotive vehicle.
The present application claims priority from Japanese Patent Application Nos. 2005-229692 and 2005-266612, the disclosures of which are incorporated herein by reference.
2. Description of the Related Art
The electric parking brake system is usually actuated by such an electric actuator as an electric motor for applying a braking force to a wheel of a vehicle while the vehicle is parking or stopping (see, for example, Japanese patent application laid-open No. 2004-51039).
As the electric parking brake is easily operational through an electrical changeover switch operated by a driver, such troublesome works as operations by a manual lever or foot stepping down operations of a brake pedal can be reduced. And, unintentional vehicle creeps due to a creeping phenomenon in an automatic transmission (hereinafter referred to as “AT creep”) or an inclination of roads etc. can be also prevented by automatic actuation thereof in accordance with vehicle conditions. Accordingly, a safety improvement is also effectively attained.
Formerly, it is well known from, for example, Japanese patent application laid-open No. 2002-242968 that the electric parking brake is provided with an inclination sensor for detecting the inclination of the vehicle and a master cylinder pressure sensor for detecting a brake fluid pressure in a main brake, and that it is judged whether a brake actuation is required in dependency on output values therefrom.
Further, it is also known from prior arts that the electric parking brake may set a braking force in dependency on the output value from the inclination sensor so as to avoid the creeping of the vehicle when the vehicle is parking on an inclined road (see, for example, Japanese patent application laid-open No. 10-24816 and Japanese patent application laid-open No. 2004-142517).
However, there may occur a malfunction of inclination sensor in such a case. Therefore, it is requested to set an adequate braking force with a plurality of sensors in order to effectively eliminate creep of a vehicle on an inclined road even if the malfunction of inclination sensor occurs.
On the other hand, it is also well known from Japanese patent application laid-open No. 2004-51039 that the electric parking brake may be provided with a changeable braking force. For example, it is known therefrom that the electric parking brake may be provided to generate a larger braking force by a switch operation than that of ordinary braking operations of a manual parking brake by a driver.
In addition, a required braking force of the electric parking brake could be changeable even if a road inclination is constant. For example, in the case of deciding the braking force in accordance with an AT creep force, an idling speed is increased (idle up) for warming up the engine in order to re-start at an engine warm-up condition just immediately after the vehicle runs, and so the required braking force becomes large due to an increase of the AT creep force.
In such a case, it is well known from, for example, Japanese patent application laid-open No. 2005-81963 that the braking force may be increased in accordance with the increase of an engine r. p. m. by detecting the idle up of the vehicle.
In other words, the electric parking brake system requires cooperation with an engine control system for controlling the engine r. p. m. etc., but for doing so, a structure of the parking brake system becomes complicated.
Furthermore, as the electric parking brake has, for example, the defect that a friction coefficient between a brake drum and a brake shoe or between a brake disc and a pad etc. changes in dependency on a temperature decrease of such brake friction materials as written above after the vehicle runs, there may occur a situation where an actual braking force becomes lower than that set beforehand.